The present invention relates to a demodulator for a digital signal transmission system employing FSK (Frequency Shift Keying) modulation, and more particularly, to an FSK demodulator having an AFC (Automatic Frequency Control) circuit.
An FSK modulation/demodulation system is the system in which digital signals such as, "0" and "1" are made to correspond to different transmission frequencies such as f.sub.0 and f.sub.1, respectively, and these frequencies are transmitted in accordance with the digital signals.
In view of the information transmission efficiency, modulation signals (base band signals) such as an NRZ (Non Return to Zero) signal are desirable as a digital signal, but this NRZ signal cannot be faithfully transmitted through a transmission system in which a D.C. component is cut off such as, the conventional FDM-FM (Frequency Division Multiplex-Frequency Modulation) transmission path, because it contains a D.C. component in its power spectrum. Consequently, heretofore, upon transmission of information employing FSK modulation, either the NRZ signal was converted, for example, into bipolar pulses not containing a D.C. component in their power spectrum and then passed through the FDM-FM transmission path at the sacrifice of the transmission efficiency, or the NRZ signal was band-suppressed and then passed through the FM transmission path which passes a D.C. component, at the sacrifice of the frequency stability. The former solution had a disadvantage that an NRZ-bipolar converter is necessitated, the construction becomes relatively complexed, and further, the chance of occurrence of bit errors at this part is so much increased. Also, the latter solution had a disadvantage that due to the band-suppression of the NRZ signal, an amplitude modulation component is produced in this signal, and because the frequency band is broadened when passing through a non-linear part, further band-suppression is repeated, resulting in occurrence of bit errors.
To stabilize the transmitting or receiving frequency, the conventional AFC circuit has a negative feedback circuit having one frequency-stable point which is formed of a frequency discriminator circuit and a voltage-controlled oscillator. Although this circuit is effective for frequency stabilization of a transmission system in which a base band signal contains no D.C. component such as the FDM-FM transmission system and for frequency stabilization of an unmodulated carrier wave, the circuit is not effective in a transmission system in which the center frequency of the modulated wave and its average frequency do not always coincide with each other as in the case of the FSK transmission system employing NRZ signals. For instance, the center frequency f.sub.x of a signal that is FSK-modulated to a frequency f.sub.0 upon "0" and to a frequency f.sub.1 upon "1" in response to a bi-level NRZ signal, is selected at f.sub.x = (f.sub.0 + f.sub.1)/2, and therefore, there is a disadvantage that when either one frequency f.sub.1 or f.sub.2 continues for a long period, the center frequency of the signal and the center frequency of the transmission path differ from each other, and thus a bit error rate is increased.